It is known that polystyrene can be foamed with nitrogen in a two stage process. In the first stage, polystyrene at room temperature is exposed to nitrogen gas in a pressurized mold. After a predetermined period of time, the polystyrene which has absorbed nitrogen gas is removed from the mold. In the second stage, the polystyrene is foamed at a temperature above its glass transition temperature. This microcellular process for foaming polystyrene is described in U.S. Pat. No. 4,473,665. A similar process using carbon dioxide as the nucleating gas has been used to foam polyethylene terephthalate (PET). The resulting PET foams are not microcellular, i.e., having cells in the 2 to 25 micrometer range as defined in U.S. Pat. No. 4,473,665. The PET foams produced had much larger cells, with diameters on the order of 100 to 300 micrometers. With respect to foaming polyethylene terephthalate, it was reported that the high solubility of carbon dioxide in polyethylene terephthalate results in a plasticization of the polymer which results in a lowering of its glass transition temperature. It was also reported that the plasticizing was believed to assist the crystallization process during the foaming process.
The composite foam structure produced by the method of the present invention is light weight, yet stronger than previous polyethylene terephthalate foams. The material can be used in applications where the strength or stiffness of the foams is inadequate.